1. Field of the Invention
The present invention relates to a method of producing hexagonal plate-like zinc oxide particles.
2. Description of the Related Art
Hexagonal plate-like zinc oxide particles are used in powder forms for a variety of purposes, such as cosmetics and paints. The particles formed into oriented sintered compacts also can be used in other purposes, such as thermoelectric materials and sputtering targets.
For example, Patent Document 1 (WO2012/147886) discloses hexagonal plate-like zinc oxide particles having a primary particle diameter of 0.01 μm or more and an aspect ratio of 2.5 or more and discloses that the hexagonal plate-like zinc oxide particles can be used as components of cosmetics, heat-dissipating fillers, heat-dissipating resin compositions, heat-dissipating grease, and heat-dissipating paint compositions. In this document, the hexagonal plate-like zinc oxide particles are produced by ripening zinc oxide microparticles in an aqueous zinc salt solution, where the zinc oxide microparticles preferably have a particle diameter of 0.005 μm or more and 0.05 μm or less. It is believed that the zinc oxide microparticles function as seed crystals.
Methods of producing hexagonal plate-like zinc oxide particles without using any seed crystal are also known. For example, Nonpatent Document 1 (G. Han et al., J. Vac. Sci. Technol. B28(2), C2C16-C2C19 (2010)) discloses a method of producing hexagonal plate-like zinc oxide particles, wherein deionized water and an aqueous 0.10 M hexamethylenetetramine (hereinafter, HMT) solution are added to a solution of 0.10 M sodium di(2-ethylhexyl)sulfosuccinate (hereinafter, AOT) in 1-butanol to produce a microemulsion; an aqueous 0.10 M Zn(NO3)2 solution is added to the microemulsion, followed by mixing by stirring; the resulting mixed solution is gradually warmed to 75° C. and is maintained at this temperature for 3 to 4 hours; and the precipitate is collected by filtration and dried to give hexagonal plate-like zinc oxide particles. Nonpatent Document 2 (G. Han et al., e-J. Surf. Sci. Nanotech. Vol. 7 (2009) 354-357) discloses a method of producing hexagonal plate-like zinc oxide particles, wherein an aqueous solution of a mixture of 0.10 M HMT and 0.10 M Zn(NO3)2 is added to a solution of 0.10 M AOT in 1-butanol, followed by stirring; the mixture is maintained at 90° C. for 12 hours in an autoclave; and the resulting white suspension is centrifuged, washed, and dried to give hexagonal plate-like zinc oxide particles.